In a small skillet, heat olive oil and 1 tsp. butter over medium heat; add onions. Sauté, stirring occasionally, until onions are soft and dark brown (about 10 min.); set aside. In a small mixing bowl, whisk together eggs and water. In a small nonstick skillet, melt remaining butter over medium heat; add egg mixture. Shake the skillet with a short forward and backward motion. Using a heat-resistant rubber spatula, pull back a little of the cooked egg around the edge and allow uncooked egg to run over until egg mixture in center is moist (15 sec.). Spread the onions, spinach and cheese down the center of the omelet. Roll 1/3 of the omelet over onto itself, then slide it out of the skillet onto a warm plate, making the second fold as the omelet falls. Serve immediately or keep warm in a 200° oven while making additional omelets.